(1) Field of the Invention
The present invention generally relates to underwater vehicle control surfaces and more particularly to a flexible control surface having a nonconstant angle of attack.
(2) Description of the Prior Art
It is well known that control surface actuator noise and flow separation induced noise created by current "rigid" control surfaces are significant sources of unwanted noise on underwater and airborne vehicles. The rigid nature of these control surfaces increases the size of the turbulent wake behind the control surface thereby generating significant flow noise levels. The flow noise is created by two mechanisms: (1) the turbulence directly radiating to the near and far field, and (2) the induced noise caused by the turbulent excitation of the control surface and the surrounding structure. The latter causes surface and structure reradiation which is the dominant flow noise source.
One source of turbulence is the gap created when a rigid control surface is pivoted to change the course of the vehicle. Upon pivoting, a gap is created between the control surface and the base member. Water flow over this gap is turbulent and creates noise. Another source of turbulence is the shed vorticity and turbulent wake created by the rigid control surface. This induces turbulent excitation of the rigid control surface causing radiation of noise. Noise is also created when turbulent flow from the wake of the control surface is ingested by the propulsors, i.e., propellers, of the vehicle.
Recent inventions have been made to address the above difficulties. In U.S. Pat. No. 5,114,104 to Cincotta et al. a control surface was disclosed having a shape memory alloy actuator embedded in an elastomeric foil shape. This configuration did not provide an adequate deflection angle because of low mechanical advantage and space considerations. U.S. Pat. No. 5,186,420 to Beauchamp et al. corrected these problems by using a shape memory alloy at the base of the control surface. The '420 device proved to be unreliable because of varying environmental temperatures. Neither the '420 nor the '104 configuration can be a retrofit control surface aboard existing underwater vehicles because of gross design differences.